Your perception of the size of an object depends
not only on how big an image it makes on your retina but also on your
perception of the distance to the object.

Material

A partner

A meter stick

To Do and Notice

Have your partner take 4 paces away from
you.

Notice how big they appear.

Hold the meter stick at arms length and use it to
measure the height of your partner.

Your partner may have a height of 20 cm measured
on the meter stick.

Have your partner take 4 more steps away from
you.

Notice how big they appear.
Are they smaller?
Are they half as tall?

Now measure their height on the meter stick.

If they were 20 cm tall, then meter stick shows
that they are 10 cm high, one-half as tall.

By holding the meter stick at arms length and
measuring the height of an object you are measuring the angular size
of the object. The angular size of an object tells you the size of
the object on your retina.

Your perception of the size of an object depends
not only on its size on your retina but also on your perception of
the distance to the object. By adjusting your perception to allow for
distance you can make a better estimate of how big the object
"actually" is.

This means that a distant object is adjusted by
your eye and brain to look bigger than it actually is.

In order to make the same size image on your
retina, a more distant object must be larger.

So What?

When you look down at people on a sidewalk 300
feet or 20 stories below you you say "they look like
ants."

Yet when you look at a football player 300 feet
away across a football stadium, you don't say, "he looks like an
ant."

You have very little experience judging distances
when looking down from a building, you have more experience judging
distance across the ground.

So you make an adjustment for size constancy when
looking at the football player, you do not make such an adjustment
when looking at the pedestrians below.

Etc.

Size Constancy provides one possible explanation
of the Moon
Illusion.